396 PERCY SLADEN TRUST EXPEDITION 



the mesenteries of larva III. are of interest, as is also the peculiar development of the 

 filaments and endodermal outgrowths in larva IV. 



1. Larva I. Zoanthina diamayita, n. sp. (PI. 23, figs. 1,2.) 



The specimen is ovoidal, 3 mm. long by 2-25 mm. in diameter ; it is a light brown 

 colour with darker brown patches. The circular groove is deep and carries long flageUse ; 

 the oral part is smaller than the ahoral, the base of which tapers somewhat. There are no 

 longitudinal depressions on the surface and no tentacles. 



The buccal orifice is circular but the stomodseal cavity is fissure-like. The stomo- 

 dseum reaches almost to the circular groove, and is conspicuously ridged over the 

 attachments of the mesenteries. All the mesenteries extend fi:-om one pole to the other, 

 passing through the narrow pedicle between the oral and aboral halves. The endoderm 

 forms a short central pillar into the aboral end of the coelenteron, which is also largely 

 obliterated by thickenings of the parietal endoderm. The mesenterial filaments are direct 

 continuations of the stomodasal ridges and extend almost to the aboral end. 



The ectoderm is a well-marked epithelium containing, (l) numerous, small, radially 

 directed nematocysts, each with a close spiral filament and, (2) deeply stained, ovoidal, 

 granular and mucous gland cells, the whole densely packed together on its surface ; 

 beneath lie numerous spherical nuclei with a few larger, possibly immature nematocysts, 

 and there is a thin nervous layer over the structureless lamella. The ectoderm both of 

 the stomodseum and of the mesenterial filaments is ciliated, with numerous granular 

 gland cells and a few mucous cells ; there are no nematocysts. 



The structureless lamella is relatively thick and contains a few isolated cells. The 

 muscles form a single layer over well developed banners in the mesenteries. Parieto- 

 basilar muscles are also present. 



The endoderm is comparatively dense with spherical nuclei near its free edge ; large 

 patches of a yellow, slightly granular substance and scattered yellow granules are found in 

 it, particularly over the mesenteries. 



Log. Lagoon, Peros Banhos, Chagos Archipelago, 9 fathoms. 



2. Larva IL. Zoanthina variabilis, n. sp. (PL 23, figs. 3, 4.) 



The two specimens are similar in appearance and size to larva I., but there are 

 indications of ridges on the surface and the aboral end is less tapering. 



The stomodfeum extends to the circular groove and is conspicuously ridged, the ridges 

 continuing on to the mesenterial filaments, which are convoluted towards the aboral end. 

 The mesenteries extend to the aboral end, and are large and conspicuous in the pedicle. 

 They are broad and thick, and almost fill up the coelenteron ; those which are mcomplete 

 are sharply bent backwards, on the sides on which their muscles lie, towards the polyp 

 wall. At the aboral end is a central column of endoderm, to which radiating endodermal 

 outgrowths from the base and sides are fused for part of its length. 



The histology of the tissues is similar to that of larva I., but the ectoderm and the 

 endoderm are much thicker ; the deeply situated, possibly immature, nematocysts in the 

 external ectoderm are very numerous. The endoderm is crowded with small round yellow 



